Cupellation

Cupellation is a metallurgical process in which ores or alloyed metals are treated under high temperatures and carefully controlled operations in order to separate noble metals, like gold and silver, from base metals like lead, copper, zinc, arsenic, antimony or bismuth, that might be present in the ore.^[1][2][3] This process is based on the principle that precious metals do not oxidise or react chemically, contrary to what happens to the base metals; so that when they are heated at high temperatures, the precious metals remain apart and the others react forming slags or other compounds.^[4]

Since the Early Bronze Age, the process of cupellation was used to obtain silver out of smelted lead ores;^[5] by the Middle Ages and the Renaissance it was one of the most common and important processes used to refine metals. By then, fire assays were used for mineral assaying, testing fresh metals such as lead and recycled metals to know their purity (jewellery) and minting. Cupellation principles have always remained the same, they change only in the amount of material processed. It is still in use today based on the same principles.^[6][7]

Cupellation Process

Large Scale Cupellation

Native silver is a rare element, although it exists as such. Most of the time it is found in nature combined with other metals and minerals that contain silver compounds, generally in the form of sulfides such as galena (lead sulfide) or cerussite (lead carbonate). So the primary production of silver was and is done by the smelting of argentiferous lead ores which are subsequently cupelled.^[8][9]

Lead melts at 327°C while silver melts at 960°C; when mixed, as in galena, the most common argentiferous lead ore, they have to be smelted at high temperatures in a reducing condition to produce argentiferous lead. The metal then has to be melted again at high temperatures of the order of 900°C or 1000°C in a hearth or blast furnace where air flow makes possible the oxidation of the lead. Lead transforms into lead oxide (PbO) known as litharge, which captures the oxides of the rest of the metals, while silver and gold remain on top of the liquid litharge. The latter is removed or absorbed by capillary action into the hearth linings.This chemical reaction may be viewed as (Ag+Cu) + Pb + O2 → (CuO+PbO) + Ag ^[10][11][12]

The base of the hearth was dug in the form of a sauce pan and covered with an inert and porous material rich in calcium or magnesium such as shells or lime or by wood or bone ash.^[13] The necessity of lining with calcareous materials was due to the fact that lead reacts with silica (clay compounds) to form lead silicate with a viscous nature that will not allow the litharge to be absorbed in the correct way. Calcareous materials do not react with lead.^[14] Some of the litharge fumes evaporate and the rest, in a liquid state are absorbed by the porous earth linings which form what it is known as litharge cakes.^[15][16]

Cupellation furnaces after Agricola, 1556/1950

Litharge cakes are usually in the shape of circular, concavo-convex cakes of approximately 15 cm in diameter and are the most common archaeological evidence of the cupellation process in the Early Bronze Age.^[17] By their chemical composition, archaeometallurgists may know what kind of ore was treated, which were the main components in it or which steps might have been followed in the process. This information might give them insights about production process, trade, social needs or economic situations among others.

Small scale cupellation

Small scale cupellation is based on the same principle as the one done in a cupellation hearth; the main difference lies in the amount of material to be tested or obtained. The minerals have to be crushed, roasted and smelted to concentrate the metallic components in order to separate the noble metals. By the Renaissance the use of the cupellation processes was diverse: assay of ores from the mines, testing the amount of silver in jewels or coins or for experimental purposes.^[18][19][20] It was carried out in small shallow recipients known as cupels.

As the main purpose of small scale cupellation was to assay and test minerals and metals, the matter to be tested has to be carefully weighed. The assays were made in the cupellation or assay furnace, which needs to have windows and bellows to ascertain that the air oxidises the lead, as well as to be sure and prepared to take away the cupel when the process is over. Pure lead has to be added to the matter being tested to guarantee the further separation of the impurities. After the litharge has been absorbed by the cupel, buttons of silver were formed and settled in the middle of the cupel.^[21] If the alloy also contained a certain amount of gold, it settled with the silver and both had to be separated by parting.^[22]

Cupels

The primary tool for small scale cupellation was the cupel. Their manufacture was made in a very careful way. They used to be small vessels shaped in the form of an inverted truncated cone, made out of bone ashes. According to Georg Agricola,^[23] the best material was obtained from burned horns of deer although fish spines could work well. Ashes have to be ground into a fine and homogeneous powder and mixed with some sticky substance to mould the cupels. Moulds were made out of brass with no bottoms so that the cupels could be taken off. A shallow depression in the centre of the cupel was made with a rounded pestle. Cupel sizes depend on the amount of material to be assayed. This same shape has been maintained until the present.

Archaeological investigations as well as archaeometallurgical analysis and written texts from the Renaissance have demonstrated the existence of different materials for their manufacture; they could be made also with mixtures of bones and wood ashes, which were not of a very high quality, or moulded with a mixture of this kind in the bottom with an upper layer of bone ashes.^[24][25][26] Different recipes depend on the expertise of the assayer or on the special purpose for which it was made (assays for minting, jewelry, testing purity of recycled material or coins). Archaeological evidence shows that at the beginnings of small scale cupellation, potsherds or clay cupels were used.^[27][28][29]

Brass moulds for making cupels.

History

The first known use of silver was in the Near East in Anatolia and Mesopotamia during the 4th and 3rd millennium BC.,^[30][31] the Early Bronze Age. Archaeological findings of silver and lead objects together with litharge pieces and slag have been studied in a variety of sites, and metallurgical analysis suggests that by then people were confidently extracting silver from lead ores so the method would have been known earlier.

During the following Iron Age, cupellation was done by fusing the debased metals with a surplus of lead, the bullion or result product of this fusion was then heated in a cupellation furnace to separate the noble metals.^[32] Mines such as Rio Tinto, near Huelva in Spain, started to be an important political and economic site for many people around the Mediterranean Sea, as well as Laurion in Greece.^[33] Around 500 B.C. control over the Laurion mines gave Athens political advantage and power in the Mediterranean so that they were able to defeat the Persians.^[34]

During the Roman times, the empire needed large quantities of lead in order to support the Roman civilization over a great territory; they searched for open lead-silver mines in whatever area they conquered. Silver coinage became the normalised media for exchange, therefore silver production and mine control gave economic and political power. In Roman times it was worth mining lead ores if their content of silver was 0.01% or more.^[35]

It is not known with certainty when or where cupellation was used as an analytical tool for the first time. One of the earliest written references of cupels may be on Theophilus Divers Ars back in the 12th century AD.^[36] In the Middle Ages there is not much information about the development of the process. Descriptions of cupels by then were very similar to those of the 16th century.^[37]

Small-scale cupellation may be considered as the most important fire assay developed in history and many think of it as the origin of chemical analysis.^[38] Most of the written evidence comes from the Renaissance in the 16th century. Georg Agricola and Lazarus Ercker, among others, wrote about the art of mining and testing the ores, as well as detailed descriptions about the cupellation process and the ways in which it should be done. Their descriptions and assumptions have been identified within diverse archaeological findings through Medieval and Renaissance Europe. By these times the amount of fire assays increased considerably, mainly because of testing the ores in the mines in order to identify the availability of its exploitation. A primary use of cupellation was related to minting activities, and it was also used in testing jewelry.^[39] Since the Renaissance, cupellation became a standardised method of analysis that has changed very little, demonstrating its efficiency. Its development certainly touched the spheres of economy, politics, warfare and power in ancient times.

Cupellation in the New World

The huge amount of Pre-Hispanic silver adornments known especially from Perú, Bolivia and Ecuador make us wonder whether the pre-Hispanic civilizations obtained the raw material from native ores or from argentiferous-lead ores. Although native silver may be available in America, it is as rare as in the Old World. From colonial texts it is known that silver mines were open in colonial times by the Spaniards from Mexico to Argentina, the main ones, being those of Tasco, Mexico and Potosí in Bolivia.

Some kind of blast furnaces called huayrachinas were described in colonial texts, as native technology furnaces used in Perú and Bolivia to smelt the ores that come from the silver mines owned by the Spaniards. Although it is not conclusive, it is believed that these kinds of furnaces may have been used before the Spanish Conquest. Ethnoarchaeological and archaeological works done at Porco – Potosí, Bolivia have suggested it.^[40]

There are no specific archaeological accounts about silver smelting or mining in the Andes prior to the Incas. However, silver and lead artefacts have been found in the Peruvian central highlands dated in the pre-Inca and Inca periods. From the presence of lead in silver artefacts, archaeologists suggest that the cupellation process might have occurred there.^[41]

See also

• Alchemy
• History of alchemy
• Archaeometry
• Georgius Agricola
• Philosopher's stone
• History of Chemistry
• Metallurgy
• Mining

References

1. ^ Rehren, Th., Martinon-Torres, M, 2003
2. ^ Bayley, J., Rehren, Th. 2007
3. ^ Craddock, P. T. 1995
4. ^ Bayley, J. 2008
5. ^ Rehren, Th., Eckstein, K. 2002
6. ^ Bayley, J. 2008
7. ^ Hoover, H. and Hoover, H. 1950[1556]
8. ^ Kassianidou, V. 2003
9. ^ Bayley, J. 2008
10. ^ Craddock, P. T. 1995:223
11. ^ Bayley, J., Crossley, D. and Ponting, M. (eds). 2008
12. ^ Pernicka, E. et al, 1998
13. ^ Bayley, J., Eckstein, K. 2006
14. ^ Kassianidou, V. 2003
15. ^ Bayley, J., Crossley, D. and Ponting, M. (eds). 2008
16. ^ Pernicka, E.,et al. 1998
17. ^ Bayley, J. 2008: 134
18. ^ Bayley, J. 2008
19. ^ Martinón-Torres, M., Rehren, Th. 2005a
20. ^ Martinón-Torres, M. et al. 2009
21. ^ Hoover, H. and Hoover, H. 1950[1556]
22. ^ Jones, D. (ed) 2001
23. ^ Hoover, H. and Hoover, H. 1950 [1556]
24. ^ Rehren, Th., Eckstein, K. 2002
25. ^ Martinón-Torres, M. and et al. 2009
26. ^ Hoover, H. and Hoover, H. 1950[1556]
27. ^ Jones, D. (ed) 2001
28. ^ Craddock, P. T. 1991
29. ^ Martinón-Torres, M., Rehren, Th. 2005b
30. ^ Pernicka, E. et al. 1998
31. ^ Karsten H. et al, 1998
32. ^ Rehren, Th., Eckstein, K 2002
33. ^ Tylecote, R.F. 1992
34. ^ http://www.ancientgreece.com/essay/v/laurion_and_thorikos/, accessed January 15, 2010
35. ^ Tylecote, R.F., 1992
36. ^ In Rehren, Th. 2003
37. ^ Martinón-Torres, M., Rehren, Th. 2005b
38. ^ Rehren, Th., Eckstein, K. 2002
39. ^ Martinón-Torres, M., Rehren, Th. 2005b
40. ^ Van Buren, M., Mills, B. 2005
41. ^ Howe, E., Petersen, U. 1994

Bibliography

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• Bayley, J. 2008 Medieval precious metal refining: archaeology and contemporary texts compared, in Martinón-Torres, M and Rehren, Th (eds) Archaeology, history and science: integrating approaches to ancient materials by. Left Coast Press: 131-150.
• Bayley, J.,Eckstein, K. 2006. Roman and medieval litharge cakes: structure and composition, in J. Pérez-Arantegui (ed) Proc. 34th Int. Symposium on Archaeometry. Institución Fernando el Católito, CSIC, Zaragoza: 145-153. (http://ifc.dpz.es/recursos/publicaciones/26/10/_ebook.pdf )
• Bayley, J., Rehren, Th. 2007. Towards a functional and Typological classification of crucibles, in La Niece, S and Craddock, P (eds) Metals and Mines. Studies in Archaeometallurgy. Archetype Books: 46-55
• Bayley, J., Crossley, D. and Ponting, M. (eds). 2008. Metals and Metalworking. A research framework for archaeometallurgy. Historical Metallurgy Society 6.
• Craddock, P. T. 1991. Mining and smelting in Antiquity, in Bowman, S. (ed), Sience and the Past, London: British Museum Press: 57-73..
• Craddock, P. T. 1995. Early metal mining and production. Edinburgh: Edinburgh University Press.
• Hoover, H. and Hoover, H. 1950 [1556]. Georgius Agricola De Re Metallica. New York: Dover.
• Howe, E., Petersen, U. 1994. Silver and Lead in late Prehistory of the Montaro Valley, Peru. In Scott, D., and Meyers P. (eds.) Archaeometry of Pre-Columbian Sites and Artifacts: 183-197. The Getty Conservation Institute.
• http://www.ancientgreece.com/essay/v/laurion_and_thorikos/, accessed January 15, 2010
• Jones, G.D. 1980. The Roman Mines at Riotinto, in The Journal of Roman Studies 70: 146-165. Society for the promotion of Roman Studies.
• Jones, D. (ed) 2001. Archaeometallurgy. Centre for Archaeological Guidelines. English Heritage publications. London.
• Karsten, H., Hauptmann, H., Wright, H., Whallon, R. 1998. Evidence of fourth millennium BC silver production at Fatmali-Kalecik, East Anatolia. in Metallurgica Antiqua: in honour of Hans-Gert Bachmann and Robert Maddin by Bachmann, H. G, Maddin, Robert, Rehren, Thilo, Hauptmann, Andreas, Muhly, James David, Deutsches Bergbau-Museum: 57-67
• Kassianidou, V. 2003. Early Extraction of Silver from Complex Polymetallic Ores, in Craddock, P.T. and Lang, J (eds) Mining and Metal production through the Ages. London, British Museum Press: 198-206
• Lechtman, H. 1976. A metallurgical site survey in the Peruvian Andes, in Journal of field Archaeology 3 (1): 1-42.
• Martinón-Torres, M., Rehren, Th. 2005a. Ceramic materials in fire assay practices: a case study of 16th-century laboratory equipment, in M. I. Prudencio, M. I. Dias and J. C. Waerenborgh (eds), Understanding people through their pottery, 139-149 (Trabalhos de Arqueologia 42). Lisbon: Instituto Portugues de Arqueologia.
• Martinón-Torres, M., Rehren, Th. 2005b. Alchemy, chemistry and metallurgy in Renaissance Europe. A wider context for fire assay remains, in Historical Metallurgy: journal of the Historical Metallurgy Society, 39(1): 14-31.
• Martinón-Torres, M., Rehren, Th., Thomas, N., Mongiatti, A. 2009. Identifying materials, recipes and choices: Some suggestions for the study of Archaeological cupels. In Giumla-Mair, A. et al., Archaeometallurgy in Europe: 1-11 Milan: AIM
• Pernicka, E., Rehren, Th., Schmitt-Strecker, S. 1998. Late Uruk silver production by cupellation at Habuba Kabira, Syria in Metallurgica Antiqua : in honour of Hans-Gert Bachmann and Robert Maddin by Bachmann, H. G, Maddin, Robert, Rehren, Thilo, Hauptmann, Andreas, Muhly, James David, Deutsches Bergbau-Museum: 123-134.
• Rehren, Th.1996. Alchemy and Fire Assay – An Analytical Approach, in Historical Metallurgy 30: 136-142.
• Rehren, Th. 2003. Crucibles as reaction vessels in ancient metallurgy, in P.T. Craddock and J. Lang (eds), Mining and Metal Production through the Ages, 207-215. London. The British Museum Press.
• Rehren, Th., Eckstein, K 2002. The development of analytical cupellation in the Middle Ages, in E Jerem and K T Biró (eds) Archaeometry 98. Proceedings of the 31 st Symposium, Budapest, April 26 – May 3, 1998 (Oxford BAR International Series 1043 – Central European Series 1), 2: 445-448.
• Rehren, Th., Schneider, J., Bartels, Chr. 1999. Medieval lead-silver smelting in the Siegerland, West Germany. In Historical Metallurgy: journal of the Historical Metallurgy Society. 33: 73-84. Sheffield: Historical Metallurgy Society.
• Tylecote, R.F. 1992. A History of Metallurgy. Second Edition Maney for the Institute of Materials. London.
• Van Buren, M., Mills, B. 2005. Huayrachinas and Tocochimbos: Traditional Smelting Technology of the Southern Andes, in Latin American Antiquity 16(1):3-25

External links

• Porco-Potosí archaeological project http://lamar.colostate.edu/~mvanbure/index.htm
• http://www.ancientgreece.com/essay/v/laurion_and_thorikos/
• http://people.hsc.edu/drjclassics/sites/thorikos/9801.htm
• http://whc.unesco.org/en/tentativelists/1789/
• http://searchworks.stanford.edu/view/993825
• http://www.gabrielbernat.es/colonia/mineria/html/potosi.html
• http://www.riotinto.com/
• http://galileo.rice.edu/Catalog/NewFiles/ercker.html
• Söderberg, A. 2011. Eyvind Skáldaspillir's silver - refining and standards in pre-monetary economies in the light of finds from Sigtuna and Gotland. Situne Dei 2011. Edberg, R. Wikström, A. (eds). Sigtuna.